This invention relates to a television horizontal automatic frequency and phase control (AFPC) loop in which the loop gain is periodically increased during the vertical deflection cycle in order to compensate for errors occurring just before or during the vertical blanking interval.
Television displays are generated by repetitively scanning an electron beam over the surface of a picture tube viewing screen to form a lighted raster area. The picture tube electron beam intensity is modulated by video signals to form images on the screen representative of the picture to be displayed. Conventional television provides a high-speed horizontal scanning in conjunction with a relatively low-speed vertical scanning. The scanning in the vertical and horizontal directions is synchronized with synchronizing (sync) signals included in a composite video signal with the video signal to be displayed. The sync signals are extracted from the composite video, and the sync signals thus extracted are used to synchronize the vertical and horizontal-direction scanning apparatus.
A sync separator for separating the horizontal synchronizing signal from the composite video includes a differentiating circuit and a threshold circuit. The differentiating circuit selectively couples signals at and above the horizontal synchronizing frequency to the threshold circuit. The threshold circuit responds to the high-frequency higher-amplitude sync signal portions of the composite video to produce a sequence of sync pulses.
The vertical synchronizing signals contained in the composite video signal are high-amplitude pulses having low-frequency components. The vertical sync signal proper has a duration of three horizontal lines. In order to maintain the flow of horizontal sync information during the vertical synchronizing interval, the vertical synchronizing pulse includes serrations by which the horizontal oscillator may be synchronized. In the NTSC television system, vertical scanning of an image is accomplished during two successive field intervals, the horizontal scanning lines of which are interlaced. Interlacing requires that the horizontal oscillator frequency be maintained in an exact relationship with the vertical frequency. In order to help the vertical sync separator maintain exact timing in extracting the vertical sync pulses, equalizing pulses are provided in the composite video during a period of three horizontal lines preceding and following the vertical synchronizing intervals. The equalizing pulses recur at twice the rate of the horizontal sync pulses. The serrations during the vertical synchronizing pulse interval also recur at twice the rate of the horizontal sync pulses.
In television systems in which the composite video signals are modulated onto a carrier and broadcast, many of the television receivers are in areas far from the transmitting station, where a weak signal can be expected. Due to the presence of unavoidable thermal noise, and also due to various forms of interference signals which may occur in the vicinity of the receiver, it may be expected that the composite video as received and the synchronizing signals derived therefrom will be intermingled with electrical noise. This electrical noise is manifested as random variation of the desired signal amplitude, and can severely perturb the operation of the display device. Commonly, noise synchronization causes vertical and horizontal jitter, or in more extreme forms "rolling" or "tearing" of the image displayed on the raster. As transmitted, the synchronizing signal pulses recur at a rate which is carefully controlled and extremely stable. Since the presence of noise obscures the synchronizing signals in a random manner, it has become common practice to obtain synchronization of the horizontal deflection circuit with the horizontal synchronizing pulse signal by the use of an oscillator, the free-running frequency of which is near the horizontal scanning frequency, and the exact frequency and phase of which is controlled in an indirect manner by a phase-lock loop (PLL) to equal the synchronizing signal frequency and phase. Thus, when any one synchronizing pulse is obscured by noise, the rate of the oscillator remains substantially unchanged, and the deflection circuits continue to receive regular deflection control pulses. Random variations in the apparent arrival time of the sync signals are averaged by the PLL loop filter, so the deflection control pulses remain in close synchronism with the video signals.
Since the PLL is a feedback system, there is an undesirable residual phase error between the oscillator signal and the synchronizing signal. High loop gain is desirable in order to minimize error, but due to imperfections in the loop components, the loop then becomes more responsive to perturbing noise. This can be offset by reducing the closed-loop bandwidth of the PLL, which may undesirably reduce transient response time. Thus, a compromise between loop gain and bandwidth is often necessary.
With the advent of integrated circuits for low-power signal processing in television devices, it has become convenient in a PLL to compare the horizontal synchronizing signals from the sync separator with a square wave as produced by the controlled horizontal oscillator rather than with a sawtooth signal. During the synchronizing pulse interval, the PLL phase detector gates a first current source which charges a storage capacitor in a first polarity when the oscillator square wave output is high, and which turns off the first current source and turns on a second current source poled to discharge the capacitor when the oscillator output is low. Thus, when the transition time of the square-wave oscillator output is centered on the synchronizing pulse, the charging and discharging currents are equal and the net capacitor voltage does not change. This maintains the oscillator frequency constant.
With the described type of phase detector, the phase detector gain and therefore the loop gain of the PLL may decrease during the equalizing and synchronizing pulse intervals. Such a decrease in gain of the PLL may be disadvantageous when rapid slewing of the horizontal oscillator frequency or phase is required during the vertical blanking interval. This may be the case, for example, when the television receiver is to be used to display information which has been recorded on a home-type video tape recorder. Such tape recorders often have a plurality of reproduction heads, each of which is mechanically scanned across the tape. In one common scheme, two heads are used, which alternately scan the tape for a duration equal to that of a vertical field. In order to avoid loss of, or breaks in the displayed information, scanning of the succeeding field is commenced by the second head substantially concurrently with the end of scanning in the first head. However, slight differences in tape tension or in the dimensions of the mechanical tape transport acting on the tape for playback compared with the tension and dimensions when the tape was recorded results in differences in the time between succeeding horizontal synchronizing pulses in the information as recorded as compared with playback, especially during the switchover between heads. This results in a discontinuity or step change in the phase of the horizontal synchronizing pulses available for synchronizing the horizontal oscillator, which step normally occurs about five horizontal lines before the end of a vertical scanning interval and the beginning of the vertical blanking interval. A high oscillator slew rate during the vertical blanking interval is necessary to conform the horizontal oscillator phase to the synchronizing signal phase after the step change, and this conformance must be complete before scanning begins for the next succeeding field.
It is known from U.S. Pat. No. 3,846,584 issued Nov. 5, 1974 to Itoh to disconnect the loop filter from the PLL for an interval immediately following the appearance of the vertical sync signal, but a decrease in PLL gain during the equalizing and vertical synchronizing pulse intervals as may be occasioned by the presence of equalizing pulses or serrations may prevent rapid slewing of the horizontal oscillator and therefore prevent accommodation of such a step change. This may result at the top of the raster in an apparent bending or tearing of vertical lines in the displayed image. Even when the synchronizing signals associated with the video to be displayed do not have a step change in phase, the decrease in PLL gain during the equalizing and vertical synchronizing pulse intervals may be disadvantageous. This may occur, for example, in those cases in which the first and second gated current sources in the described type of phase detector have unequal amplitudes. Unequal charge and discharge currents results in a progressive change in the horizontal oscillator control signal and may result in driving the oscillator off-frequency during the equalizing and vertical synchronizing intervals in which the PLL gain is low. If the loop gain is increased during this interval as suggested by Itoh, the oscillator may drift off-frequency very quickly, and there may then be insufficient time remaining before the beginning of the next following scanning interval for correction, resulting in an apparent bending or tearing of vertical lines in the displayed image.